A train passes overhead as people march to the Port of Oakland to block an Israeli ship. Photograph: Steve Rhodes/Demotix/Corbis

A group of activists returned to the Port of Oakland in California for a second day on Sunday to picket outside dock entrances in an effort to prevent an Israeli ship from unloading its cargo in protest of recent violence in Gaza.

The ship, the Zim Piraeus, operated by Zim Integrated Shipping Services Ltd, had come into port earlier that day and was waiting to unload. But dockworkers reporting for the evening shift did not cross the picket lines, creating further delay.

“They’re honoring our community picket and they haven’t been crossing,” said organizer Lara Kiswani, who said she was in contact with members of the workers’ union, International Longshore and Warehouse Union Local 10.

On Saturday, protesters had set out to stage early-morning pickets as part of an action dubbed Block the Boat for Gaza, organized by a Bay Area coalition that included some 70 organizations. While the August 16 march and rally drew a large crowd to the port, the blockade didn’t go forward since the vessel remained out at sea.

The Guardian sought comment from Zim Integrated Shipping Services and the Port of Oakland but had not heard back by press time.

Organizers had been monitoring the movements of the ship online throughout the day on Sunday, and put out a text alert at 5.10pm urging people to return for a second blockade attempt once it became clear that the ship was headed into dock.

“We didn’t expect to be out here today until an hour before the ILWU workers were coming to work,” explained organizer Sharif Zakout. The picketing started shortly after 5pm, he said. By 6.30pm, as more people arrived, groups of between 20 to 50 protesters were seen circling outside four different entrance gates. Throughout the evening, police officers stood calmly in lines in front of the dock entrances. The protest came to an end at about 8pm.

It was unclear whether the ship would be unloaded Monday. “They may unload it in the morning, they may not,” Kiswani said. She added that Zim vessels tend to dock at the Port of Oakland every Saturday. “Our intention is to really build a movement here … our goal in the long run is for the workers themselves to refuse to unload that ship, stand with us, and take a position against Israeli apartheid.”